1. Field of the Invention
This invention generally relates to non-contact automated data collection systems in which a portable token card or fare tag, commonly referred to as a "smart card," is used with a stationary target terminal to exchange financial and other data in a mass-transit fare transaction system. More particularly, the invention relates to an improved data collection system having collision resolution features to prevent more than one fare tag from establishing a communication link with a single target.
2. Description of the Related Art
Smart card technology has been effectively used in mass-transit systems. In such an application, the smart card retains a fare value representative of funds available for use by its holder. As the smart card holder uses his card for transportation services, value is subtracted from the smart card in accordance with the applicable fare, or added in exchange for proper consideration.
The utilization of smart card technology in mass-transit applications reduces waste and increases efficiency by replacing paper ticket, mechanical coin, and token reading devices. Waste is reduced through the elimination of fare tickets. Efficiency is enhanced by the increased transaction speed and the ease of use of automated non-contact data collection systems for admittance to and departure from the transit system. A typical smart card transaction takes place within a 100 millisecond time period, roughly seven times faster than the time it takes to pass a paper ticket through a standard mechanical transport. For admittance or departure, the smart card need be merely presented in the proximity of the target provided in the stationary target terminal for the fare transaction to take place. Moreover, since data is transmitted via a radio frequency ("RF") field, no physical contact between a smart card and target is required. The smart card may even be retained in a storage area, such as a purse or wallet, as long as it is presented in the proximity of the target.
The capabilities of the smart card system have also been exploited in multi-modal mass-transit systems. In such a system, a smart card is designed to integrate payment schemes for various forms of ground transportation and related services. For instance, smart card technology has been utilized as a common means of payment for local rail, bus, and parking services provided by a particular local transit system. One such system developed by the assignee of this application is disclosed in International Application Number PCT/US92/08892, entitled "Non-Contact Automatic Fare Collection System," filed Oct. 19, 1992, and published May 13, 1993 as WO 93/09516.
A demonstration system generally applying the teachings of this application is currently operating in the Washington Metro Area Transit Authority (WMATA) mass-transit system for rail service, ground transportation (buses), and parking lots. In the WMATA system currently in use, fare data is transmitted between the stationary target terminal and a smart card, referred to herein as a fare tag, via a RF field. A standard target terminal consists of a target and a remotely located controlling computer. The target includes a modulator/demodulator and an antenna designed to transmit and receive, via an RF field with a carrier signal frequency of 2 MHz, a message modulated upon the carrier. During operation, the target emits a continuous RF field designed to evoke a response from a fare tag located in the general proximity of the target. Once a fare tag is brought in range, it is powered by the target's RF transmission and its responds with a message to the target. The target antenna receives the RF transmission from the responding fare tag, demodulates the message and conveys it to the device to which it is connected. The device determines if the message it has received is in the proper message format and, if so, it responds with a message. The fare tag receives the response and determines if the response is in the proper message format. If the response is in the proper message format, the fare tag responds by communicating with the controlling computer, and the appropriate fare data is read from the tag. The controlling computer then calculates the resulting fare value and that value is transferred and written to the tag's memory.
This demonstration system did not include any means or method for resolving message collisions because, as a result of its low signal power transmission characteristics, the target could only power one fare tag at a time. If two tags were presented in the proximity of the target, neither tag's internal circuitry would reach the required threshold voltage to generate and transmit a message. However, at higher power levels, message collisions can occur if more than one fare tag is presented in the proximity of a target and those fare tags are activated. Once activated, the tags simultaneously transmit information to the target. The simultaneously transmitted information collides and creates a signal which the target cannot recognize. Accordingly, there is a need for an effective collision resolution system that will prevent message collisions from multiple tags in higher power systems.